Hydration of simple double bond functions in organic molecules to form alcohols is well known in the art and is readily accomplished by acid catalysis. Hydration of symmetrical substituted molecules such as .alpha., .beta.-unsaturated dicarboxylic acids is relatively more difficult and generally requires special reaction conditions. The acid catalyzed hydrations of maleic acid and fumaric acid to produce malic acid are well known examples of such processes.
In contrast to maleic and fumaric acid, .alpha., .beta.-unsaturated dicarboxylic acids containing additional carboxyl groups one or more carbon atoms away from the double bond, such as aconitic acid, are very resistant to acid catalyzed hydration, and under such conditions either preferentially undergo decarboxylation to produce a mixture of dicarboxylic acids or do not react at all.
In addition to the acid catalysts used for hydrating simple .alpha., .beta.-unsaturated dicarboxylic acids such as maleic and fumaric acids the art also recognizes the use of alkali metal hydroxides for this purpose as taught by F. Loydl, Ann. 192 80 (1878) and Van't Hoff, Jr., Ber. 18 2713 (1885) which are incorporated herein by reference. R. Berg, in U.S. Pat. No. 3,128,287, particularly column 1, lines 68-72 and column 2, lines 1-2, which is also incorporated herein by reference, teaches that maleic acid is converted into about an equal mixture of malic acid and oxydisuccinic acid by heating with excess calcium hydroxide or magnesium hydroxide in aqueous medium. The Berg reference also teaches that when strontium hydroxide and barium hydroxide are utilized in his process, the maleic acid is converted almost quantitatively into malic acid. Ger. Offen. 2,220,295 discloses the reaction of aconitic acid with glycolic acid in the presence of excess calcium hydroxide to form an addition product. However, there appears to be no references in the literature regarding the behavior of aconitic acid towards the action of excess aqueous alkali in the absence of other reagents. We have found that heating an aqueous solution of aconitic acid with an excess of an alkali metal hydroxide such as potassium hydroxide cleaves the molcule to produce a mixture of the corresponding alkali metal salts of acetic acid and itaconic acid as well as other degradation products. This cleavage can be explained in part in terms of a reverse aldol reaction with the intermediate hydration product as has been described for the hydrolytic cleavage of double bonds in general by Patai, in "The Chemistry of the Alkenes," Interscience Publishers, 1964, page 548, which is incorporated herein by reference.
In summary, neither the acid nor alkaline catalyzed hydration of aconitic acid to form citric acid and/or isocitric acid or their salts has been taught by the prior art. If a suitable hydration process could be devised for aconitic acid, it would afford a new and relatively simple route to citric acid and/or isocitric acid which have well known commercial uses such as, but not limited to, food acidulants and flavors and metal cleaners.